In the data processing art, data is usually stored in a magnetic disc drive in binary digital form on magnetic recording surfaces, such as the surfaces of a disc pack. The disc pack in one form includes a number of magnetic discs fastened to a rotatable hub in vertical spaced positions, with each disc having recording surfaces on both sides. The disc pack is constructed for use with a disc file unit which includes a motor for rotating the pack, and which also includes a bank of electromagnetic transducer heads mounted on a positioning mechanism which moves the transducer heads radially in unison from one radial track to another on the disc surfaces. Each transducer head in the head assembly is magnetically coupled to a different one of the disc recording surfaces, and each transducer head is adapted to record new data on the radial tracks on the associated surface, or to read data from the radial tracks on the surface under the control of appropriate electronic equipment.
As mentioned above, the binary data is recorded on the several surfaces of the magnetic discs in a circular track format, with each disc surface having a plurality of concentric tracks of data. In order to write or read data from any particular track on any particular disc surface, an appropriate mechanism is provided for setting the particular transducer in operative relationship with the particular track, and a servo system is also provided for maintaining the transducer centered on the track.
One type of magnetic disc drive has one or more disc surfaces for data storage; and one disc surface, usually near the center of the disc pack, which is dedicated to servo tracks exclusively. A special servo read head transducer reads the servo tracks on the dedicated surface. The associated servo system responds to the signals from the servo head to position the arm or carriage carrying the servo head so that the servo head is maintained directly over the correct servo tracks and, therefore, the data heads which are mounted on the same arm are correctly maintained in alignment with the corresponding data tracks.
Another type of drive has recently been developed which has embedded servo tracks that take the place of the dedicated servo tracks discussed above. Embedded servo tracks are written on the data surfaces of the disc pack. They are not continuous tracks, but they consist of bursts of servo information written between data sectors on each data track. Each data head reads data during the sector data time and reads servo information between successive sector data times. The servo information is sampled when that information is read by the particular data head, and is held during the sector data times.
The dedicated servo and embedded servo magnetic disc drives discussed above usually have movable discs which are rotatably driven about a particular axis of rotation, and transducers which are held on an arm or a carriage in a stationary position with respect to the discs, until the heads are moved by the arm or carriage from one radial track to another. Another type of magnetic disc drive has recently been developed using low flying heads and which are sealed to exclude dust particles. This latter type of disc drive is called a "Winchester" drive. The Winchester drives have remarkably high data storage capacity. In the Winchester drive the head carriage is positioned by a closed loop servo system.
All the servo tracks, either dedicated or embedded, operate in pairs. A data track location is defined as the center between a pair of servo tracks. The servo tracks of each pair are usually designated EVEN and ODD. In operation, the servo head in the dedicated servo type, or the corresponding data head in the embedded servo type, straddles the space between the EVEN and ODD servo tracks. The servo head operates to position the arm or carriage so that it reads equal amounts of even and odd information from the two servo tracks. If the drive has a dedicated servo surface, it is usual to obtain the write clock and index, and sometimes the sector marks, from the servo tracks. In the case of the embedded servos, the clock cannot be obtained from the servo tracks, because the servo tracks are not continuous. Therefore, in the case of the embedded servo tracks, a separate clock head and an associated closed master clock track must be provided in order to write the servo information on the disc. However, during normal operation a self-clocking data track is used and no separate clock is required.
Nearly all the servo tracks on magnetic disc drives, dedicated or embedded, are closed. That is, the information in the servo tracks has no gap or discontinuity at the origin. Therefore, the servo information in the servo tracks must be written from a master clock track which, likewise, is closed. Writing a closed master clock track on a magnetic disc of acceptable quality is extremely difficult. The system disclosed in U.S. Pat. No. 4,131,920 which issued Dec. 26, 1978 in the name of the present inventor is one of a very few systems which are available for that purpose.
In the case of magnetic disc drives with removable discs and dedicated servo surfaces, the precise mechanical position of the servo tracks is important. Each track must be written at precisely the correct radius to assure compatibility of discs between drives. Removable discs for drives of this type have dedicated servo surfaces recorded by special instruments which embody precise mechanical positioners, such as laser interferometers. One such instrument is disclosed, for example, in U.S. Pat. No. 4,068,268 which issued Jan. 10, 1978 to Tom Y. Idemoto et al.
In the case of magnetic disc drives with non-removable discs, or using embedded servos, the requirement for the mechanical position of the tracks is less severe than in the case of the drives with the removable discs. Presently, the servo tracks for the magnetic disc drives with non-removable discs are also written using mechanical positioners, but of a less expensive type than required for the removable disc type of drive with the dedicated servo surface.
In either case, the servo writing procedure is to position the arm or carriage mechanically at the extreme inside track or outside track, and then to write one of the two servo tracks, and then to move the arm or carriage one track width, and write the other servo track, and to repeat this process over and over again until all the servo tracks have been written. Although the procedure is accomplished automatically, as described in the Idemoto et al patent, it is slow and requires bulky and expensive equipment.
The system disclosed in the Berger U.S. Pat. No. 4,131,920 may be considered as a basis for a unit for writing the servo tracks. This is because a basic requirement for any such unit is the ability to produce a closed master clock track. However, the system described in the Berger patent does not have suitable internal logic or operating controls to operate automatically in order to write a series of servo tracks, and to position the servo head between each writing. Moreover, it does not have sufficient pattern selection capability to write the servo information used in most magnetic disc drives, since eight patterns are usually required, and the Berger system can select only six.
The system of the present invention finds particular utility in conjunction with the "Winchester" magnetic disc drive described above using embedded servo tracks. The system is capable of writing servo tracks on the Winchester magnetic disc drives automatically and without the use of mechanical positioners. This means that the servo tracks may be written without breaking the seal of the Winchester drive, and the operation may be performed without the need for a clean room enviroment. However, the system of the invention, as mentioned above, is also capable of operating in conjunction with precision mechanical positioners for writing servo tracks on the movable disc dedicated servo surface magnetic disc drives of the Winchester and other types. The system of the invention can be portable, and it is capable of writing a closed clock track of high quality. The system has a pattern storage capability, as well as the controls and sequencing necessary to write servo tracks automatically. Moreover, the system has the capacity to store internally a large number of different servo patterns, which may be selected by the operator. These patterns are stored in programmable read-only memories (PROMS) which are easily programmable for new requirements.